Third generation (3G) mobile systems, such as, for instance, universal mobile telecommunication systems (UMTS) standardized within the third generation partnership project (3GPP) have been based on wideband code division multiple access (WCDMA) radio access technology. Today, 3G systems are being deployed on a broad scale all around the world. After enhancing this technology by introducing high-speed downlink packet access (HSDPA) and an enhanced uplink, also referred to as high-speed uplink packet access (HSUPA), the next major step in evolution of the UMTS standard has brought the combination of orthogonal frequency division multiplexing (OFDM) for the downlink and single carrier frequency division multiplexing access (SC-FDMA) for the uplink. This system has been named long term evolution (LTE) since it has been intended to cope with future technology evolutions.
The LTE system represents efficient packet based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. The detailed system requirements are given in 3GPP TR 25.913, “Requirements for evolved UTRA (E-UTRA) and evolved UTRAN (E-UTRAN),” v8.0.0, January 2009, (available at http://www.3gpp.org/ and incorporated herein by reference). The Downlink will support data modulation schemes QPSK, 16QAM, and 64QAM and the Uplink will support BPSK, QPSK, 8PSK and 16QAM.
LTE's network access is to be extremely flexible, using a number of defined channel bandwidths between 1.25 and 20 MHz, contrasted with UMTS terrestrial radio access (UTRA) fixed 5 MHz channels. Spectral efficiency is increased by up to four-fold compared with UTRA, and improvements in architecture and signalling reduce round-trip latency. Multiple Input/Multiple Output (MIMO) antenna technology should enable 10 times as many users per cell as 3GPP's original WCDMA radio access technology. To suit as many frequency band allocation arrangements as possible, both paired (frequency division duplex FDD) and unpaired (time division duplex TDD) band operation is supported. LTE can co-exist with earlier 3GPP radio technologies, even in adjacent channels, and calls can be handed over to and from all 3GPP's previous radio access technologies.
FIG. 1 illustrates structure of a component carrier in LTE Release 8. The downlink component carrier of the 3GPP LTE Release 8 is sub-divided in the time-frequency domain in so-called sub-frames 100 each of which is divided into two downlink slots 110 and 120 corresponding to a time period Tslot. The first downlink slot comprises a control channel region within the first OFDM symbol(s). Each sub-frame consists of a given number of OFDM symbols in the time domain, each OFDM symbol spanning over the entire bandwidth of the component carrier.
FIG. 2 is an example illustrating further details of LTE resources. In particular, the smallest unit of resources that can be assigned by a scheduler is a resource block also called physical resource block (PRB). A PRB 210 is defined as NsymbDL consecutive OFDM symbols in the time domain and NscRB sub-carriers in the frequency domain. In practice, the downlink resources are assigned in resource block pairs. A resource block pair consists of two resource blocks. It spans NscRB sub-carriers in the frequency domain and the entire 2·NsymbDL modulation symbols of the sub-frame in the time domain. NsymbDL may be either 6 or 7 resulting in either 12 or 14 OFDM symbols in total. Consequently, a physical resource block 210 consists of NsymbDL×NscRB resource elements 220 corresponding to one slot in the time domain and 180 kHz in the frequency domain (further details on the downlink resource grid can be found, for example, in 3GPP TS 36.211, “Evolved universal terrestrial radio access (E-UTRA); physical channels and modulations (Release 8)”, version 8.9.0, December 2009, Section 6.2, available at http://www.3gpp.org. which is incorporated herein by reference).
The number of physical resource blocks NRBDL in downlink depends on the downlink transmission bandwidth configured in the cell and is at present defined in LTE as being from the interval of 6 to 110 PRBs.
The data are mapped onto physical resource blocks by means of pairs of virtual resource blocks. A pair of virtual resource blocks is mapped onto a pair of physical resource blocks. The following two types of virtual resource blocks are defined according to their mapping on the physical resource blocks in LTE downlink:
Localised Virtual Resource Block (LVRB)
Distributed Virtual Resource Block (DVRB)
In the localised transmission mode using the localised VRBs, the eNB has full control which and how many resource blocks are used, and should use this control usually to pick resource blocks that result in a large spectral efficiency. In most mobile communication systems, this results in adjacent physical resource blocks or multiple clusters of adjacent physical resource blocks for the transmission to a single user equipment, because the radio channel is coherent in the frequency domain, implying that if one physical resource block offers a large spectral efficiency, then it is very likely that an adjacent physical resource block offers a similarly large spectral efficiency. In the distributed transmission mode using the distributed VRBs, the physical resource blocks carrying data for the same UE are distributed across the frequency band in order to hit at least some physical resource blocks that offer a sufficiently large spectral efficiency, thereby obtaining frequency diversity.
In 3GPP LTE Release 8 there is only one component carrier in uplink and downlink. Downlink control signalling is basically carried by the following three physical channels:
Physical control format indicator channel (PCFICH) for indicating the number of OFDM symbols used for control signalling in a sub-frame (i.e. the size of the control channel region);
Physical hybrid ARQ indicator channel (PHICH) for carrying the downlink ACK/NACK associated with uplink data transmission; and
Physical downlink control channel (PDCCH) for carrying downlink scheduling assignments and uplink scheduling assignments.
The PCFICH is sent from a known position within the control signalling region of a downlink sub-frame using a known pre-defined modulation and coding scheme. The user equipment decodes the PCFICH in order to obtain information about a size of the control signalling region in a sub-frame, for instance, the number of OFDM symbols. If the user equipment (UE) is unable to decode the PCFICH or if it obtains an erroneous PCFICH value, it will not be able to correctly decode the L1/L2 control signalling (PDCCH) comprised in the control signalling region, which may result in losing all resource assignments contained therein.
The PDCCH carries control information, such as, for instance, scheduling grants for allocating resources for downlink or uplink data transmission. A physical control channel is transmitted on an aggregation of one or several consecutive control channel elements (CCEs). Each CCE corresponds to a set of resource elements grouped to so-called resource element groups (REG). A control channel element typically corresponds to 9 resource element groups. A scheduling grant on PDCCH is defined based on control channel elements (CCE). Resource element groups are used for defining the mapping of control channels to resource elements. Each REG consists of four consecutive resource elements excluding reference signals within the same OFDM symbol. REGs exist in the first one to four OFDM symbols within one sub-frame. The PDCCH for the user equipment is transmitted on the first of either one, two or three OFDM symbols according to PCFICH within a sub-frame.
Another logical unit used in mapping of data onto physical resources in 3GPP LTE Release 8 (and later releases) is a resource block group (RBG). A resource block group is a set of consecutive (in frequency) physical resource blocks. The concept of RBG provides a possibility of addressing particular RBGs for the purpose of indicating a position of resources allocated for a receiving node (e.g. UE), in order to minimise the overhead for such an indication, thereby decreasing the control overhead to data ratio for a transmission. The size of RBG is currently specified to be 1, 2, 3, or 4, depending on the system bandwidth, in particular, on NRBDL. Further details of RBG mapping for PDCCH in LTE Release 8 may be found in 3GPP TS 36.213 “Evolved Universal terrestrial Radio Access (E-UTRA); Physical layer procedures”, v8.8.0, September 2009, Section 7.1.6.1, freely available at http://www.3gpp.org/ and incorporated herein by reference.
Physical downlink shared channel (PDSCH) is used to transport user data. PDSCH is mapped to the remaining OFDM symbols within one sub-frame after PDCCH. The PDSCH resources allocated for one UE are in the units of resource block for each sub-frame.
FIG. 3 shows an exemplary mapping of PDCCH and PDSCH within a sub-frame. The first two OFDM symbols form a control channel region (PDCCH region) and are used for L1/L2 control signalling. The remaining twelve OFDM symbols form data channel region (PDSCH region) and are used for data. Within a resource block pairs of all sub-frames, cell-specific reference signals, so-called common reference signals (CRS), are transmitted on one or several antenna ports 0 to 3. In the example of FIG. 3, the CRS are transmitted from two antenna ports: R0 and R1.
Moreover, the sub-frame also includes UE-specific reference signals, so-called demodulation reference signals (DM-RS) used by the user equipment for demodulating the PDSCH. The DM-RS are only transmitted within the resource blocks in which the PDSCH is allocated for a certain user equipment. In order to support multiple input/multiple output (MIMO) with DM-RS, four DM-RS layers are defined meaning that at most, MIMO of four layers is supported. In this example, in FIG. 3, DM-RS layer 1, 2, 3 and 4 are corresponding to MIMO layer 1, 2, 3 and 4.
One of the key features of LTE is the possibility to transmit multicast or broadcast data from multiple cells over a synchronized single frequency network which is known as multimedia broadcast single frequency network (MBSFN) operation. In MBSFN operation, UE receives and combines synchronized signals from multiple cells. To facilitate this, UE needs to perform a separate channel estimation based on an MBSFN reference signal. In order to avoid mixing the MBSFN reference signal and normal reference signal in the same sub-frame, certain sub-frames known as MBSFN sub-frames are reserved from MBSFN transmission. The structure of an MBSFN sub-frame is shown in FIG. 4 up to two of the first OFDM symbols are reserved for non-MBSFN transmission and the remaining OFDM symbols are used for MBSFN transmission. In the first up to two OFDM symbols, PDCCH for uplink resource assignments and PHICH can be transmitted and the cell-specific reference signal is the same as non-MBSFN transmission sub-frames. The particular pattern of MBSFN sub-frames in one cell is broadcasted in the system information of the cell. UEs not capable of receiving MBSFN will decode the first up to two OFDM symbols and ignore the remaining OFDM symbols. MBSFN sub-frame configuration supports both 10 ms and 40 ms periodicity. However, sub-frames with number 0, 4, 5 and 9 cannot be configured as MBSFN sub-frames. FIG. 4 illustrates the format of an MBSFN subframe.
The PDCCH information sent on the L1/L2 control signalling may be separated into the shared control information and dedicated control information. The frequency spectrum for IMT-advanced was decided at the World Radio Communication Conference (WRC-07) in November 2008. However, the actual available frequency bandwidth may differ for each region or country. The enhancement of LTE standardized by 3GPP is called LTE-advanced (LTE-A) and has been approved as the subject matter of Release 10. LTE-A Release 10 employs carrier aggregation according to which two or more component carriers as defined for LTE Release 8 are aggregated in order to support wider transmission bandwidth, for instance, transmission bandwidth up to 100 MHz. It is commonly assumed that the single component carrier does not exceed a bandwidth of 20 MHz. A terminal may simultaneously receive and/or transmit on one or multiple component carriers depending on its capabilities.
Another key feature of the LTE-A is providing relaying functionality by means of introducing relay nodes to the UTRAN architecture of 3GPP LTE-A. Relaying is considered for LTE-A as a tool for improving the coverage of high data rates, group mobility, temporary network deployment, the cell edge throughput and/or to provide coverage in new areas.
A relay node is wirelessly connected to radio access network via a donor cell. Depending on the relaying strategy, a relay node may be part of the donor cell or, alternatively, may control the cells on its own. In case the relay node is a part of the donor cell, the relay node does not have a cell identity on its own, however, may still have a relay ID. In the case the relay node controls cells on its own, it controls one or several cells and a unique physical layer cell identity is provided in each of the cells controlled by the relay. At least, “type 1” relay nodes will be a part of 3GPP LTE-A. A “type 1” relay node is a relaying node characterized by the following:
The relay node controls cells each of which appears to a user equipment as a separate cell distinct from the donor cell.
The cells should have its own physical cell ID as defined in LTE Release 8 and the relay node shall transmit its own synchronization channels, reference symbols etc.
Regarding the single cell operation, the UE should receive scheduling information and HARQ feedback directly from the relay node and send its controlled information (acknowledgments, channel quality indications, scheduling requests) to the relay node.
The relay node should appear as a 3GPP LTE compliant eNodeB to 3GPP LTE compliant user equipment in order to support the backward compatibility.
The relay node should appear differently to the 3GPP LTE eNodeB in order to allow for further performance enhancements to the 3GPP LTE-A compliant user equipments.
FIG. 5 illustrates an example 3GPP LTE-A network structure using relay nodes. A donor eNodeB (d-eNB) 510 directly serves a user equipment UE1 515 and a relay node (RN) 520 which further serves UE2 525. The link between donor eNodeB 510 and the relay node 520 is typically referred to as relay backhaul uplink/downlink. The link between the relay node 520 and user equipment 525 attached to the relay node (also denoted r-UEs) is called (relay) access link.
The donor eNodeB transmits L1/L2 control and data to the micro-user equipment UE1 515 and also to a relay node 520 which further transmits the L1/L2 control and data to the relay-user equipment UE2 525. The relay node may operate in a so-called time multiplexing mode, in which transmission and reception operation cannot be performed at the same time. In particular, if the link from eNodeB 510 to relay node 520 operates in the same frequency spectrum as the link from relay node 520 to UE2 525, due to the relay transmitter causing interference to its own receiver, simultaneous eNodeB-to-relay node and relay node-to-UE transmissions on the same frequency resources may not be possible unless sufficient isolation of the outgoing and incoming signals is provided. Thus, when relay node 520 transmits to donor eNodeB 510, it cannot, at the same time, receive from UEs 525 attached to the relay node. Similarly, when a relay node 520 receives data from donor eNodeB, it cannot transmit data to UEs 525 attached to the relay node. Thus, there is a sub-frame partitioning between relay backhaul link and relay access link.
Regarding the support of relay nodes, in 3GPP it has currently been agreed that:
Relay backhaul downlink sub-frames during which eNodeB to relay downlink backhaul transmission is configured, are semi-statically assigned.
Relay backhaul uplink sub-frames during which relay-to-eNodeB uplink backhaul transmission is configured are semi-statically assigned or implicitly derived by HARQ timing from relay backhaul downlink sub-frames.
In relay backhaul downlink sub-frames, a relay node will transmit to donor eNodeB and consequently r-UEs are not supposed to expect receiving any data from the relay node. In order to support backward compatibility for UEs that are not aware of their attachment to a relay node (such as Release 8 UEs for which a relay node appears to be a standard eNodeB), the relay node configures backhaul downlink sub-frames as MBSFN sub-frames.
In the following, a network configuration as shown in FIG. 5 is assumed for exemplary purposes. The donor eNodeB transmits L1/L2 control and data to the macro-user equipment (UE1) and 510 also to the relay (relay node) 520, and the relay node 520 transmits L1/L2 control and data to the relay-user equipment (UE2) 525. Further assuming that the relay node operates in a time-duplexing mode, i.e. transmission and reception operation are not performed at the same time, we arrive at a non-exhaustive entity behavior over time as shown in FIG. 6. Whenever the relay node is in “transmit” mode, UE2 needs to receive the L1/L2 control channel and physical downlink shared channel (PDSCH), while when the relay node is in “receive” mode, i.e. it is receiving L1/L2 control channel and PDSCH from the Node B, it cannot transmit to UE2 and therefore UE2 cannot receive any information from the relay node in such a sub-frame. In the case that the UE2 is not aware that it is attached to a relay node (for instance, a Release-8 UE), the relay node 520 has to behave as a normal (e-)NodeB. As will be understood by those skilled in the art, in a communication system without relay node any user equipment can always assume that at least the L1/L2 control signal is present in every sub-frame. In order to support such a user equipment in operation beneath a relay node, the relay node should therefore pretend such an expected behavior in all sub-frames.
As shown in FIGS. 3 and 4, each downlink sub-frame consists of two parts, control channel region and data region. FIG. 7 illustrates an example of configuring MBSFN frames on relay access link in situation, in which relay backhaul transmission takes place. Each subframe comprises a control data portion 710, 720 and a data portion 730, 740. The first OFDM symbols 720 in an MBSFN subframe are used by the relay node 520 to transmit control symbols to the r-UEs 525. In the remaining part of the sub-frame, the relay node may receive data 740 from the donor eNodeB 510. Thus, there cannot be any transmission from the relay node 520 to the r-UE 525 in the same sub-frame. The r-UE receives the first up to two OFDM control symbols and ignores the remaining part of the sub-frame. Non-MBSFN sub-frames are transmitted from the relay node 520 to the r-UE 525 and the control symbols 710 as well as the data symbols 730 are processed by the r-UE 525. An MBSFN sub-frame can be configured for every 10 ms on every 40 ms. Thus, the relay backhaul downlink sub-frames also support both 10 ms and 40 ms configurations. Similarly to the MBSFN sub-frame configuration, the relay backhaul downlink sub-frames cannot be configured at sub-frames with #0, #4, #5 and #9.
Since MBSFN sub-frames are configured at relay nodes as downlink backhaul downlink sub-frames, the relay node cannot receive PDCCH from the donor eNodeB. Therefore, a new physical control channel (R-PDCCH) is used to dynamically or “semi-persistently” assign resources within the semi-statically assigned sub-frames for the downlink and uplink backhaul data. The downlink backhaul data is transmitted on a new physical data channel (R-PDSCH) and the uplink backhaul data is transmitted on a new physical data channel (R-PUSCH). The R-PDCCH(s) for the relay node is/are mapped to an R-PDCCH region within the PDSCH region of the sub-frame. The relay node expects to receive R-PDCCH within the region of the sub-frame. In time domain, the R-PDCCH region spans the configured downlink backhaul sub-frames. In frequency domain, the R-PDCCH region exists on certain resource blocks preconfigured for the relay node by higher layer signalling. Regarding the design and use of an R-PDCCH region within a sub-frame, the following characteristics have been agreed so far in standardization:
R-PDCCH is assigned PRBs for transmission semi-statically. Moreover, the set of resources to be currently used for R-PDCCH transmission within the above semi-statically assigned PRBs may vary dynamically, between sub-frames.
The dynamically configurable resources may cover the full set of OFDM symbols available for the backhaul link or may be constrained to their sub-set.
The resources that are not used for R-PDCCH within the semi-statically assigned PRBs may be used to carry R-PDSCH or PDSCH.
In case of MBSFN sub-frames, the relay node transmits control signals to the r-UEs. Then, it can become necessary to switch transmitting to receiving mode so that the relay node may receive data transmitted by the donor eNodeB within the same sub-frame. In addition to this gap, the propagation delay for the signal between the donor eNodeB and the relay node has to be taken into account. Thus, the R-PDCCH is first transmitted starting from an OFDM symbol which, within the sub-frame, is late enough in order for a relay node to receive it.
The mapping of R-PDCCH on the physical resources may be performed either in a frequency distributed manner or in a frequency localised manner.
The interleaving of R-PDCCH within the limited number of PRBs can achieve diversity gain and, at the same time, limit the number of PRBs wasted.
In non-MBSFN sub-frames, Release 10 DM-RS is used when DM-RS are configured by ENodeB. Otherwise, Release 8 CRS are used. In MBSFN sub-frames, Release 10 DM-RS are used.
R-PDCCH can be used for assigning downlink grant or uplink grant for the backhaul link. The boundary of downlink grant search space and uplink grant search space is a slot boundary of the sub-frame. In particular, the downlink grant is only transmitted in the first slot and the uplink grant is only transmitted in the second slot of the sub-frame.
No interleaving is applied when demodulating with DM-RS. When demodulating with CRS, both REG level interleaving and no interleaving are supported.
Based on the above agreement, there are basically three different options for configuration of the R-PDCCH search space:
Frequency localised non-interleaved R-PDCCH,
Frequency distributed non-interleaved R-PDCCH, and
REG-level interleaved R-PDCCH.
For the REG-level interleaved R-PDCCH, the Release 8 PDCCH search space scheme will be reused within the semi-statically configured PRBs for R-PDCCH (the so-called R-PDCCH virtual bandwidth). For the non-interleaved R-PDCCH, the Release 8 PDCCH search space concept of randomizing the positions of PDCCH candidates for different aggregation levels across the whole bandwidth could theoretically be applied, but would not facilitate the benefit that the candidates can be in positions which can be freely assigned by the eNodeB. This would, in turn, make it impossible to exploit the full frequency-selective scheduling benefit for the control channel.